The manufacturing process of flat panel display substrates requires specific sized glass substrates capable of being processed in standard production equipment. To obtain substrates having the proper size, mechanical scoring and breaking processes, or a laser scoring techniques are employed. Each of these sizing methods requires edge finishing. The finishing process involves grinding and/or polishing the edges to remove sharp edges and other defects that may degrade the strength and durability of the substrate. Furthermore, there are many processing steps that require handling in the manufacturing of an LCD panel. Thus, glass substrates used for Liquid Crystal Displays (LCD) require an edge that is sufficiently durable for mechanical handling and contact.
The finished edges are created by grinding the unfinished edge with an abrasive metal grinding wheel. In conventional systems, the glass substrate is disposed on a chuck and advanced through a series of grinding positions. Each position is equipped with a different abrasive grinding wheel based on the coarseness/fineness of the grit disposed on the wheel. The finishing process is complete after the glass substrate traverses each grinding position. However, when the glass is not properly aligned relative to the grinding wheel, the quality of the finished glass substrate is degraded. In particular, glass misalignment can adversely impact the dimensional accuracy of the glass. Second, glass misalignment may cause inferior edge quality, which usually results in a substrate of inferior strength. Accordingly, substrate breakage may occur during LCD processing steps. Further exacerbating the problems discussed above, is the demand for larger and larger display sizes. This demand, and the benefits derived from economies of scale, are driving AMLCD manufacturers to process larger display substrates. It is therefore critical that larger display substrates are provided having the requisite edge quality, dimensional accuracy, and strength.
There are three approaches that are being considered to address the above stated issues. In one approach, substrate manufacturers are evaluating grinding systems that offer improved alignment accuracy. Unfortunately, since LCD manufacturers are using larger and larger substrates, alignment tolerances become much more critical when the size of the substrate increases. Accurate alignment is more of a necessity because small skew angles translate into larger errors when larger substrates are being processed. One drawback to this approach relates to the fact that while alignment tools may be acquired having the requisite precision, the accuracy cannot be maintained over time due to wear.
In another approach that has been considered, grinding systems may be employed that compensate for lack of alignment accuracy by removing more material. Typically, edge finishing grinding systems need only remove approximately 100 microns of material. The concept is to provide a larger substrate and remove the right amount of material to meet dimensional requirements. One way to accomplish this is to use a system that includes multiple grinding steps. This translates into more grinding spindles and more grinding wheels. One drawback to this approach is the capital expense of the additional processing equipment. Further, once the equipment is obtained, more equipment requires more maintenance. Another way to remove more material is to employ coarser grinding wheels. Unfortunately, this option is not attractive because a rougher finish has a greater propensity for substrate breakage.
Yet another way to remove more material is to reduce the speed at which substrates traverse the finishing system. Unfortunately, this approach reduces production capacity and the ground edge quality. Further, increased capital expenditures would be required if the production volume is to be maintained.
In yet another approach that has been considered, a self-aligning grinding system may be used that tracks the substrate edge. The pressure feed grinding approach applies a predetermined force normal to the edge of the substrate. The grinding wheel moves, or tracks, with the instantaneous position of the edge by rotating about a pivot element. Because grinding wheel position is determined by the position of the substrate edge, the resultant substrate product has improved dimensional accuracy, relative to conventionally ground substrates. Unfortunately, there is a drawback to this technique as well. The cylindrical pivot employed in conventional pressure feed systems includes mechanical bearings. In order to overcome the frictional force of these mechanical bearings, a normal force of approximately 16N must be applied. This force exceeds the strength of the glass substrate and breakage will occur if that force is applied. While the pressure feed grinding approach appears to be promising, it cannot be employed unless the aforementioned problems are overcome.
In light of the foregoing, it is desirable to provide an edge finishing apparatus that is configured to remove a precise amount of glass and yet maintain the edge quality. It is also desirable to provide an edge finishing apparatus having improved dimensional accuracy. Furthermore, the edge finishing apparatus should finish the edge of a glass in a timely manner without degrading the desired strength and edge quality attributes of the glass. What is needed is a pressure feed grinding apparatus that provides the above described features while overcoming the limitations of conventional pressure feed grinding systems discussed above.